Conventionally, a well-known concentration measurement device is integrated into a gas supply line for supplying a raw material gas such as an organic metal (MO) gas and the like to a semiconductor manufacturing equipment, to measure the concentration of the gas in the gas supply line.
In this type of concentration measurement device, concentration is obtained from absorbance that is measured by a photodetector detecting light absorbed by a gas while the gas passing through inside a measurement cell provided in a gas supply line, into which a light having a predetermined wavelength is incident from a light source.
However, errors in the measurement concentration occur under the influence of the ambient environment such as temperature, and the fluctuation in light emission intensity of the light source or light receiving intensity of a photodetector (light receiving element) or the like. Calibration is necessary to maintain the measurement accuracy, but the gas supply line of the semiconductor manufacturing equipment needs to be calibrated while avoiding contact with outside to prevent contamination.
For that reason, an in-line system concentration measurement device has been suggested to include a measurement cell providing a fluid to be measured, a calibration cell having similar characteristics to the measurement cell providing a fluid for calibration, a light source projecting light switchable to one of the two cells selectively, a light receiver receiving light having passed through inside the cell (for example, patent document 1, etc.).